Re: 6lowpan raw socket problems

[Alexander Aring <alex.aring@gmail.com>]

I forget this,

On Thu, Sep 18, 2014 at 06:30:08PM +0200, Alexander Aring wrote:
> On Thu, Sep 18, 2014 at 04:54:53PM +0100, Simon Vincent wrote:
> > It looks like in 6lowpan_iphc.c lowpan_header_compress the original ipv6
> > header is removed and the compressed header is attached.
> > 
> > These four lines are responsible.
> >     skb_pull(skb, sizeof(struct ipv6hdr));
> >     skb_reset_transport_header(skb);
> >     memcpy(skb_push(skb, hc06_ptr - head), head, hc06_ptr - head);
> >     skb_reset_network_header(skb);
> > 
> > I don't think we can do this as the skb is being used in other parts of the
> > ip stack. Hence when the ipv6 header is read elsewhere the addresses become
> > corrupt as they have been overwritten by the 6lowpan compressed header.
> > 
> > Any ideas on how to fix this?
> > 
> yes, but I don't believe that this makes trouble. <--- or only makes
> trouble by replacing data, see below.
> 
> It's called by a callback of header_ops [0].
> 
> This is for generating the mac header with address information from
> neighbor discovery cache (mainly destination address) and source
> addresse (mainly netdev->dev_addr).
> 
> Another example of this function is ethernet. [1]
> 
> On [1] you will se that the ethernet header will created there.
> 
> - Get data from skb for ethhdr (ethernet header)
>   struct ethhdr *eth = (struct ethhdr *)skb_push(skb, ETH_HLEN);
> 
> - memcpy(eth->h_source, saddr, ETH_ALEN); <-- source address
> 
> - memcpy(eth->h_dest, daddr, ETH_ALEN); <-- destination address.
> 
> 
> So they using the callback there to manipulate the skb here.
> 
> Another idea is that, maybe we can ADD data but not REPLACING existing
> data with that. I don't know right now.
> 
> 
> But I moved this handling out of the create callback of header_ops. This
> also fix the issue by running wireshark&co on a lowpan interface.
> 
> What I did is only to save the address information in the reserved room
> of skb in this callback. [2]
> 
> Then running replacing header in the xmit callback of lowpan device. It
> looks very different now! I have also splitted the lowpan implementation
> in three files "main.c, tx.c, rx.c". Please note that. This code is part
> of the rework and I want to fix the wireshark&co issue there.
> 
> I don't have a solution for this right now which applies on current
> mainline, sorry! But the xmit callback is 100% secure by replacing skb
> header data.
> 
> If you like you can try to apply it on mainline. What I said is that
> this also fix the IPv6 capturing on a lowpan interface. I want also say
> to you that I added a comment there "TODO ask david or marc if this run
> into trouble", because I am not sure if the reserved skb room can be
> overwritten sometimes, then we stuck into the same issue. (Worked on my
> side currently), but I also don't do much raw socket RPL messages. ;-)
> 
> What I know is that we can't use skb->cb here to save the information,
> this would be overwritten by traffic control.
> 

See also:

https://github.com/linux-wpan/linux-wpan-next/commit/b86faef5523ddb451e8e4784ea1a49a852901289

but don't trust the diff, I think there was some copy&paste bug.

What I do there in a small sentence "Only save address information in
"lowpan_header_create", do IPv6 -> 6LoWPAN replacing in "lowpan_xmit".

Hope it's understandable what I did there and why, because wireshark
capture after lowpan_header_create but before lowpan_xmit. And then we
seeing the 6LoWPAN header on wireshark which is wrong on a lowpan interface.

- Alex